Chemistry

Chemistry

1. Atoms and it's charge

   • Proton +1
     • Neutron 0
       • Electron -1
   • In an atom the number of electrons are equal to the amount of a proton. Atoms have no overall electrical charge.
     • All atoms of a particular element have the same number of protons atoms have different elements have different number of protons.
       • The number of protons in an atom is called it's atomic number (proton and electron number) the electrons in an atom occupy the lowest available energy levels (inner shell)
   • Compounds are substances in which atoms of 2 or more elements are not just mixed together but chemically combined. Chemical bonding involves either transferring or sharing electrons in the highest occupied energy levels (shells) of atoms.
     • When atoms form chemical bonds by transferring electrons they form ions. Atoms that lose electrons become positively charged ions. Atoms that gains an electron becomes negatively charged ions. Ions have the electronic structure of a noble gas (balanced)
   • Elements in group 1 of the periodic table, the alkali metals have similar chemical properties. They all react with non-metal elements to form ionic compounds in which the metal ion has a single positive charge.
     • Elements in group 7, the halogens have similar chemical properties they react with the alkali metals to form ionic compounds in which the halide ions have a single negative charge.
       • ionic compound is a giant structure of ions ionic compounds are held together by strong forces of attraction between oppositely charged ions theses forces act in all directions in the lattice and this is called ionic bonding.
         • When atoms share pair of electrons they form covalent bonds. These bonds between atoms are strong some covalently bonded substances consist of simple molecules such as H2, HCL, H2O.
           • Metals consist of giant structures of atoms arranged in a regular pattern the electrons in the highest occupied energy levels(outer shells) of metal atoms are delocalised and so free to move through the whole structure this corresponds to a structure of positive ions with electrons between ions holding them together by strong electrostatic forces.
         • When atoms share pairs of electrons they from covalent bonds these bonds between atoms are strong. Metal consists of giant structures of atoms arranged in a regular pattern the electrons in the highest occupied energy levels (outer shells)of metal atoms are delocalised so are free to move throughout the whole structure this corresponds to a structure of positive ions with electrons between ions holding them together by strong electrostatic attraction.

2. Substances

   • Substances that consist of simple molecules are gasses, liquids and solids that have relatively low melting and boiling points.
     • Substances that consist of simple molecules have only weak forces between the molecules (intermolecular forces). it is these intermolecular forces that are overcome not the covalent bonds when the substance boils or melts
       • Substances that consist of simple molecules do not conduct electricity because the molecules do not have an overall electric charge.
         • Ionic compounds have regular structures (giant ionic lattices) in which there are strong electrostatic forces in all directions between oppositely charged ions these compound have high melting and boiling points
           • When melted or dissolved in water ionic compounds conduct electricity as the ions are free to move and carry the current.
             • Atoms that share electrons can also form giant structures or macromolecules diamond and graphite(forms of carbon) and silicon dioxide are examples of giant covalent structures of atoms. all these atoms in these structures are linked o other atoms by strong covalent bonds so they have very high melting points.
               • In diamond each carbon atom forms four covalent bonds with other carbon atoms in a giant covalent structure which makes diamond very hard
                 • In graphite each carbon atom bonds to 3 others forming layers the layers are free to slide over each other and so graphite is soft and slippery. in Graphite one electron frm each carbon atom is delocalised these delocalised electrons allow it to conduct heat and electricity
                   • The layers of atoms in metals are able to slide over each other and so metals can be bent and shaped

3. The relative masses of protons, neutrons and electrons are- Proton-1, Neutron-1, Electron very small(0)

   • The total number of protons and neutrons in an atom is called mass number
     • Atoms of the same element can have different numbers of neutrons theses atoms are called isotopes of that element.
       • The relative atomic mass of an element (Ar) compares the mass of atoms of the element with the 12c isotope.
         • The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in formula
           • The relative formula mass of a substance in grams is known as one mole of that substance. The percentage of an element in a compound can be calculated from the relative mass of the element in the formula and the relative formula mass of the compound
             • The masses of reactants and products can be calculated from balanced symbol equations
               • Even though no atoms are gained or lost in an chemical reaction it is not always possible to obtain the calculated amount of product because- the reaction may not go to completion because it is reversible, some of the product may be lost when it is separated from the reaction mixture and some of the reactants may react in ways different to the expected reaction
                 • The amount of product obtained is known as yield percentage
                   • In some chemical reactions the products of the reaction can react to produce the original reactants such reactants are called reversible reactions

4. Rate of reaction

   • The rate of a chemical reaction can be found by measuring the amount of a reactant used or the amount of product formed
     • The rate of a chemical reaction increases if the temperature increases, If the concentration of dissolved reactants or the pressure of gasses increases, If solid reactants are in smaller pieces(greater surface area) and if a catalyst is used.
       • Chemical reactions can only occur when reacting particles collide with each other and with sufficient energy the minimum amount of energy particles must have to react is called the activation energy.
         • Increasing the temperature increases the speed of the reacting particles so that they collide more frequently and more energetically this increases the rate of reaction.
           • Increasing the concentration of reactants in solutions and increasing the pressure of reacting gases also increases the frequency of collisions and so increases the rate of reaction
             • Concentrations of solutions are given in moles per cubic decimetre(mol/dm3)
               • Equal volumes of gases at the same temperature and pressure contain the same number of molecules
                 • Catalysts change the rate of chemical reactions but are not used up during the reaction Different reactions need different catalysts.
                   • Catalysts are important in increasing the rates of chemical reactions used in industrial processes to reduce cos

5. Exothermic/ Endothermic

   • When chemical reaction occurs energy is transferred to it's surroudings
     • An exothermic reaction is one that transfers energy often as heat to the surroundings examples of exothermic reactions include combustion many oxidation reactions and neutralisation.
       • An endothermic reaction is one that takes in energy often as heat from the surroundings for e.g. thermal decomposition.
         • If a reversible reaction is exothermic in one direction it is endothermic in the opposite direction the same amount of energy is transferred in each case.
           • When a reversible reaction occurs in a closed system equilibrium is reached when the reactions occur at exactly the same rate in each direction
             • The relative amounts of all reacting substances at equilibrium depend on the conditions of the reaction
               • If the temperature is raised the yield from the endothermic reaction increases and the yield from the exothermic reaction decreases
                 • if the temperature is lowered the yield from the endothermic reaction decreases and the yield from exothermic reaction increases.
                   • In gaseous reactions an increase in pressure will favour the reaction that produces the least number of molecules as shown by the symbol equation for that reaction
                     • These factors together with reaction rates are important when determining the optimum conditions in industrial processes including the haber process
                       • Sustainable development as well as economic reasons to minimize energy requirements ad energy wasted in industrial processes non-vigorous conditions mean less energy is used and less is released into the environment

6. Ionic compounds have many uses and can provide other substances electrolysis is used to produce alkali's and elements such as chlorine and hydrogen.

   • oxidation reduction reactions do not just involve oxygen soluble salts can be made from acids, insoluble salts can be made from solutions of ions.
     • Passing a current through ionic substances that are molten or in solution breaks them down into elements this process is called electrolysis.
       • During electrolysis positive charged ions move to the negative electrode and negative charged ions move to the positive electrode
         • At the negative electrode positively charged ions gain electrons(reduction) and at the positive electrode negatively charged ions lose electrons(oxidation)
           • if there is a mixture of ions the products formed depend on the reactivity of the elements involved.
             • the electrolysis of sodium chloride solution produces hydrogen and chlorine sodium hydroxide solution is also produced these are important reagents for the chemical industry.
               • Copper can be purified by electrolysis using a positive electrode made of the impure copper and a negative electrode of pure copper of pure copper in a solution containing copper ions.
                 • Insoluble salts can be made by mixing appropriate solutions of ions so that a precipitate is formed. precipitation is used to remove unwanted ions from solutions for e.g. removing ions from drinking water.
                   • Soluble salts can be made from acids by reacting them with: Metals- not all metals are suitable some are too reactive and others not reactive enough.
                     • They can also be made by reacting them with insoluble bases- the base is added to the acid until no more will react and the excess solid is filtered off.
                       • Finally can be reacted wit Alkali's an indicator can be used to show when the acid and alkali have completely reacted to produce a salt solution.
                         • Salt solutions can be crystalized to produce solid salt. metal oxides and hydroxides are bases soluble hydroxides are called alkali's
                           • The particular salt produced in any reaction between an acid, base or alkali depend on- the acid used(hydrochloric acid produces chloride, nitric acid produces nitrates and sulphuric acid produces sulphates) and it depends on the metal in the base or alkali
                             • ammonia dissolves in water and makes alkaline solution used to produce ammonium salts and ammonium salts Is important as fertilizers.
                               • Hydrogen ions H+ make solutions acidic and hydroxide ions OH- make solutions alkaline the pH scale is a measure of the acidity or alkalinity of a solution.
                                 • In neutralisation reactions, hydrogen ions react with hydroxide ions to produce water this reaction can be presented in a equation
                                   • H+(aq) + OH-(aq) - H2O(l)